Particulate material heating apparatus and method

ABSTRACT

Apparatus for firing products of combustion into an overburden of finely divided particulate material incorporates a perforated plate covering a combustion chamber. Openings in the perforated plate define a total open area which is a relatively small percentage of the total plate area. This forms a plenum in the combustion chamber which is characterized by a substantially uniform pressure adjacent the perforated plate. The perforations are typically orders of magnitude wider than the average width of the individual particles forming the overburden to facilitate flow of the combustion products. The perforation depth is several multiples of the perforation width to inhibit the passage of the material through the perforations.

Unlted States Patent 1 1 1111 3,910,755

Syska 1 Oct. 7, 1975 [5 PARTICULATE MATERIAL HEATING 3,361,539 l/1968 Pyzel 432/58 APPARATUS AND METHOD 3,539,293 11/1970 Boucraut et al. 432/15 3,711,956 1/1973 Brauer et a1 432/15 Inventor: r w J- SySka, Marblehead, 3,765,101 10/1973 Avery 432/58 Mass.

[73] Assignee: Thermo Electron Corporation, Primary Examiner-Johfl Camby Waltham, Ma Assistant Examiner-Henry C. Yuen Attorney, Agent, or Firm.lames L. Neal [22] 1-1led2 July 29, 1974 Appl. No.: 492,368

Related U.S. Application Data Continuation of Ser. No. 399,675, Sept. 21, 1973, abandoned, which is a continuation of Ser. No. 273,955, July 21, 1972, abandoned.

Apparatus for firing products of combustion into an overburden of finely divided particulate material incorporates a perforated plate covering a combustion chamber. Openings in the perforated plate define a total open area which is a relatively small percentage of the total plate area. This forms a plenum in the combustion chamber which is characterized by a substantially uniform pressure adjacent the perforated plate. The perforations are typically orders of magnitude wider than the average width of the individual particles forming the overburden to facilitate flow of the combustion products. The perforation depth is several multiples of the perforation width to inhibit the passage of the material through the perforations.

ABSTRACT 4 Claims, 2 Drawing Figures PARTICULATE MATERIAL HEATING APPARATUS AND METHOD This is a continuation of'co-pending application Ser. No. 399,675, filed Sept. 21, 1973, which is a continuation of application Ser. No. 273,955, filed July 21, 1972, both now abandoned.

BACKGROUND OF THE INVENTION Heat treatment of finely divided particulate material in various kiln structures, such as lime or cement kilns, requires that the particles of finely divided material be brought into the best possible heat transfer relationship to the heat source. Various devices for establishing this heat transfer relationship have been used. Attempts to obtain efficient transfer are fraught with difficulty because the finely divided material tends to enter areas of the apparatus where its presence is not acceptable. This problem is particularly evident when a source of combustion products is submerged beneath an overburden of finely divided material. However, burners submerged beneath a burden of finely divided particulate material are in a highly advantageous position to effect efficient heat transfer. Common practice has been to use equipment effecting relatively inefficient heat transfer because that equipment would appropriately confine finely divided material being heated.

A primary object of this invention is to provide good heat transfer between a heat source and finely divided particulate material.

Another object of this invention is to provide means for supporting finely divided particulate material and passing products of combustion therein while preventingthe passage of such material through the support during either operating or non-operating conditions.

A further object of this invention is to provide a sur face for supporting an overburden of particulate material and to pass products of combustion through the surface uniformly over its area.

BRIEF SUMMARY OF THE INVENTION This invention provides apparatus for firing jets of combustion products into an overburden of finely di vided particulate material. A perforated plate supports the particulate material over a combustion zone so that combustion products may pass through perforations in the plate directly into the particulate material. The chief unique features of the invention reside in the distribution and configuration of perforations in the plate and the manner in which such distribution and configuration relate to the average size of particles forming the finely divided particulate material. In particular, the width of the opening formed by the perforation in the surface of the perforated plate should be substantially greater, and may be several orders of magnitude greater, than the characteristic width of the particles forming the particulate material; for example, the particle width might be 0.010 inches while the diameter of a round perforation might be from 0.125 to 0.250 inches. The thickness of the perforated plate and thereby the depth of the perforation should be several multiples of the opening width formed by the perforation. The relatively high ratio of perforation length to perforation width inhibits the passage of particulate material through the perforation both during operating and non-operating conditions. During non-operating conditions, the particulate material which begins to pass through the perforations will encounter an amount of friction sufficient to cause the material to lodge in the perforations to form a solid mass, and prevent further entry of material into the perforations. During operating conditions, the combustion products passing from the combustion zone through the relatively long perforations fluidize the finely divided material lodged in the perforations and continously blow it therefrom.

A relatively small percent of the surface area of the perforated plate should be occupied by the perforations. However, the open area in the plate should be great enough that, for a given flow rate, the pressure drop across the plate does not create an unacceptable amount of dust. Further, an excessive pressure drop across the plate can result in excessively high velocity jets which blow not only dust, but all material away from the plate. On the other hand, the open area of the plate should be small enough to maintain the plenum formed in the combustion chamber at substantially uniform pressure. When the pressure in the plenum is uniform, the flow of products of combustion through the plate will be uniform over the plate area. If pressure in the plenum is not uniform, there will be non-uniform heat output over the plate area. A typical result of excessive perforation of the plate is greater flow of products of combustion through the center of the plate than around its periphery.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows the apparatus of this invention installed in a kiln structure; and

FIG. 2 shows a perforated plate usable with the apparatus of FIG. 1.

DETAILED DESCRIPTION OF THE DRAWINGS Referring to FIG. I, there is shown a high temperature refractory support 10 forming combustion chambers 12 which are enclosed by a pair of plates 14 having perforations 16 therein. Each of the combustion chambers l2 communicates through a neck 18 with a burner head 20. The burner head 20 is connected with one or more reactant inlets 22 and 24 for receiving a combustible mixture (for example, natural gas and air) which is forced through the system under pressure by a blower means 26. Products of combustion produced in the combustion chamber 12 issue as jets from the perforation 16. The entire assembly may be covered by means forming a wall 30 to confine a material being heated. The wall 30 may be of any suitable material such as refractory brick.

The burner heads 20 may be of any type which mixes fuel and air intimately to give essentially complete combustion below the perforated plate 14 and substantially no combustion above the perforated plate. The burner heads 20 and the blower means 26 are commercially available hardware. Suitable burners are obtainable from Eclipse Fuel Engineering Company of Rockford, Ill.; Pyronics, Inc. of Cleveland, Ohio; and North American Manufacturing Company, 4455 East 71st St., Cleveland, Ohio. One suitable burner is the North American Manufacturing Company burner No. 4832-4, Flat Flame Burner.

The above described apparatus for firing products of combustion into an overburden of finely divided particulate material should meet the following requirements:

a. the perforated plate 16 must not fail under operating temperature and pressure conditions;

b. there must be uniform heat release over the surface of the perforated plate;

c. the apparatus must operate whether covered with finely divided particulate material or not, and when under partial coverage;

d. the finely divided particulate material must not pass through the perforated plate when the combustion or air supply is cut off.

When installed in a kiln or the like, the entire assembly is located in the kiln wall so that the inside surface of the perforated plate 14 is substantially flush with a kiln wall 11 formed by the supports 10.

In order for the apparatus to operate in a cement kiln with an overburden of cement mix, or in a similar environment, the perforated plate must be made of a material which can withstand the high temperatures generated by the flame plus that re-radiated by the hot material mix, for example, a ceramic plate. The material must also withstand the thermal shock of cold mix falling unto the hot plate plus the pressure of the overburden and the pressure drop across the plate.

In order to achieve a uniform heat release over the entire plate surface, the perforations preferably occupy not substantially less than one percent or substantially greater than ten percent of the total plate area. This range does not constitute an absolute limit and the manner in which the percentage of open plate area may be varied can best be understood by reference to the factors ultimately to be controlled. For a given flow rate, the pressure drop across the perforated plate 16 is a function of the percent of open plate area. This parameter is valued, as described below, to create a plenum in the combustion chamber 12 which is of substantially uniform pressure throughout. To achieve this resuit the frictional loss, or pressure drop, across the combustion chamber 12 should be much less than the pressure drop across the perforated plate 14. For example, the pressure drop occurring across the perforated plate 14 may be ten times greater than the pressure drop occurring between the neck 18 and the surface of 15 of the perforated plate 14. If the open area of the plate becomes too great, this relative difference in the pressure drop cannot be maintained and the pressure in the combustion chamber 12, and thereby the heat output, will vary across the surface of the plate 14. Additionally, the pressure drop across the plate is calibrated to be substantially greater than the maximum anticipated pressure drop across the overburden of finely divided material so that the weight of the material will not block or substantially reduce the flow of combustion products through the perforations. On the other hand, if the percent of open area in the surface of the perforated plate 14 is too low, there may be an excessive pressure drop across the plate for the given flow rate. Under optimum operating conditions, jets issuing from all perforations 16 will be of the same height. If the pressure drop across the plate I4 is too low, the jets will vary in heat intensity and height. For apparatus constructed along the lines of those shown in FIG. 1, the optimum percent of open plate area appears to lie in the range of one percent to three percent.

In reference to FIG. 2, there may be seen one simple and acceptable pattern of perforations for a plate 14. Any perforation pattern suitable to give specific appli cation may be used. The perforations in FIG. 2 are shown in a preferred uniform distribution but they need not necessarily be so distributed. FIG. 2 shows perforations aligned substantially normal to the surfaces of the plate 14.

Jet issuing from perforations 16 should be well developed and stable so that penetration of the finely divided material will be uniform. For this reason, the ratio of perforation length (l) to perforation diameter or width (d) is preferably not substantially less than five. For the apparatus shown in FIGS. 1 and 2, best results have been obtained with l/d ratios not substantially less than eight or substantially more than twelve. However, successful results were obtained with an l/d ratio of 16. These jet conditions successfully fiuidize the particulate material in the perforations l6 and produce a fluidizing effect upon the overburden which is controllable in its magnitude as desired. Control may conveniently be produced as a function of the output of the blower means 26.

The apparatus produces a level of dusting during operation which is unobejctionable for most applications, including cement kilns. However, concial perforations having the large end of the core directed toward the overburden of particulate material of the type shown in phantom lines and designated 16, affords a substantial further reduction in dusting.

A further requirement is that the combustion chamber not fill up with the finely divided material when combustion is stopped. Perforations 16 in the plate 14 are typically round and have diameters much larger than the average'particle size of the finely divided material. The perforation diameters may be one-eighth to one-fourth inch while, in the case of a cement kiln, the average particle size of the finely divided material is usually less than 200 mesh (e.g., 0.0029 in.). An l/d ratio of five successfully inhibits the passage of such particles into the combustion chamber 16. Particularly good results are obtained in the apparatus of the preferred embodiment with an l/d ratio of substantially eight or more.

The apparatus operates to provide substantially complete combustion in the combustion chamber 12 and to issue substantially uniform jets of products of combustion from each of the perforations 16 whether or not an overburden of particulate material covers the plates 14, thus causing it to be insensitive to variations in overburden load during operation and to overburden load conditions at start-up and shut-down.

For very high temperature operation, such as in eement kilns, the plate 14 maybbe constructed of silicon carbide, aluminum oxide, zirconium oxide or of some combination of the three. However, in any given application, the plate may be of any material which will withstand the temperature and pressure conditions to which it is subjected.

A comparison of the effect of l=/d ratio on the ability of various perforated plates to inhibit passage of finely divided particulate material can be seen from the table which appears below.

Plate Inhibited Hole Dia. Spacing N01 7( Plate Thick l/d Open Material (d) in. in. Holes Area (I) in. Passage 1 No. 0.125 0.25 420 3.58 0.05 0.4 2 No 0.045 0.03 0.6 3 Yes (L250 I00 3.40 2.00 8.0

Continued Plate Inhibited Hole Dia. Spacing No. 7! Plate Thick l/d Material (d) in. in. Holes Area (I) in.

Passage 4 Yes 0.125 225 2.17 2.00 16.0 5 Yes 0. l 25 1.00 121 1.03 1.50 12.0 6 Yes 0.125 1.00 12] 1.03 1.50 l2.0

The present invention has been described with reference to preferred embodiments. It should be understood, however, that modifications may be made by those skilled in the art without departing from the scope of the invention.

I claim: 1. ln a process of treating finely ground material having an average particle size not in excess of 200 mesh (0.0029 inches) utilizing a bifurcated chamber divided by a perforated plate, an operating cycle which comprises the steps of:

firing combustion products upward through substantially uniformly distributed openings in said plate not substantially less than 0.125 inches across or substantially greater than 0.250 inches across having upper width dimensions at least as large as corresponding lower width dimensions, a ratio of opening length to opening width of at least 5 and providing at least 1 percent and not substantially in excess of 3 percent open area in said plate;

creating a substantially uniform pressure drop at all openings through said plate means during firing conditions, thereby producing substantially uniform jets of combustion products above said plate means;

introducing quantities of material to be treated into a treatment region above said plate means to form an overburden upon said plate means;

2. In the process of claim 1, the operating cycle wherein said firing step comprises:

burning a fluid fuel in a combustion chamber immediately beneath and adjacent said perforated plate means; and forcing combustion products through said plate means by blower means.

3. In the process of claim 2, the operating cycle further comprising the step of controlling the fluidizing effect as a function of the output of said blower means.

4. In the process of claim 3, the operating cycle wherein said step of creating a substantially uniform pressure drop comprises the step of creating a pressure drop across said plate approximately ten times greater than the pressure drop across said combustion chamber. 

1. In a process of treating finely ground material having an average particle size not in excess of 200 mesh (0.0029 inches) utilizing a bifurcated chamber divided by a perforated plate, an operating cycle which comprises the steps of: firing combustion products upward through substantially uniformly distributed openings in said plate not substantially less than 0.125 inches across or substantially greater than 0.250 inches across having upper width dimensions at least as large as corresponding lower width dimensions, a ratio of opening length to oPening width of at least 5 and providing at least 1 percent and not substantially in excess of 3 percent open area in said plate; creating a substantially uniform pressure drop at all openings through said plate means during firing conditions, thereby producing substantially uniform jets of combustion products above said plate means; introducing quantities of material to be treated into a treatment region above said plate means to form an overburden upon said plate means; by means of said firing step, heat treating such material and at least partially fluidizing the same; subsequent to the above said steps, shutting down said firing; permitting said overburden to settle upon said plate; and obstructing the downward passage through said plate means of said material to be treated during periods of non-firing by lodging action of said materials within said openings along the lengths thereof.
 2. In the process of claim 1, the operating cycle wherein said firing step comprises: burning a fluid fuel in a combustion chamber immediately beneath and adjacent said perforated plate means; and forcing combustion products through said plate means by blower means.
 3. In the process of claim 2, the operating cycle further comprising the step of controlling the fluidizing effect as a function of the output of said blower means.
 4. In the process of claim 3, the operating cycle wherein said step of creating a substantially uniform pressure drop comprises the step of creating a pressure drop across said plate approximately ten times greater than the pressure drop across said combustion chamber. 